1. Field of the Invention
Embodiments of the present invention relate to a semiconductor device and a method for manufacturing a semiconductor device.
2. Discussion of the Background
An internal-combustion engine ignition device has been known which generates a high voltage from a low voltage and ignites a fuel-air mixture at a predetermined timing. FIG. 5 is a circuit diagram illustrating the structure of an internal-combustion engine ignition device 700. The structure and operation of the internal-combustion engine ignition device 700 will be described with reference to FIG. 5. The internal-combustion engine ignition device 700 includes an engine control unit (ECU) 51, an ignition IC 52, an ignition coil 56, an ignition plug 60, and a voltage source 59. The ignition IC 52 includes a protective diode 53, an insulated gate bipolar transistor (IGBT) 54, and resistors 55 and 61. The ignition coil 56 includes a primary coil 57 and a secondary coil 58.
The voltage source 59 supplies a constant voltage (for example, about 14 V) and is connected to one terminal of the primary coil 57 in the ignition coil 56. The other terminal of the primary coil 57 is connected to a C terminal of the ignition IC 52 (a collector electrode of the IGBT 54). An E terminal of the ignition IC 52 (an emitter electrode of the IGBT 54) is connected to the ground, and a G terminal of the ignition IC 52 (a gate electrode of the IGBT 54) is connected to the ECU 51. The ECU 51 has a function of transmitting a signal which controls the turn-on (short circuit) and turn-off (open) of the IGBT 54 forming the ignition IC 52 to the G terminal of the ignition IC 52. For example, a voltage of 5 V is applied to the G terminal of the ignition IC 52 to short-circuit the IGBT 54 of the ignition IC 52. In contrast, a voltage of 0 V is applied to the G terminal of the ignition IC 52 to open the IGBT 54 of the ignition IC 52.
Specifically, when an on-signal output from the ECU 51 is input to the G terminal of the ignition IC 52, the IGBT 54 of the ignition IC 52 is short-circuited and a collector current Ic starts to flow from the voltage source 59 to the C terminal of the ignition IC 52 through the primary coil 57 of the ignition coil 56. In contrast, when an off-signal output from the ECU 51 is input to the G terminal of the ignition IC 52, the IGBT 54 of the ignition IC 52 is opened and the collector current Ic is rapidly reduced. The rapid change in the collector current Ic causes a rapid increase in the voltage between both ends of the primary coil 57. Similarly, the voltage between both ends of the secondary coil 58 increases to tens of kilovolts (for example, 30 kV) and is applied to the ignition plug 60. The ignition plug 60 discharges when the applied voltage reaches a desired value.
Next, the protective diode 53 forming the ignition IC 52 will be described. When a surge voltage of hundreds of volts (for example, 400 V) is applied to the C terminal of the ignition IC 52, an initial surge current flows from the C terminal of the ignition IC 52 to the gate of the IGBT 54 through the protective diode 53 (zener diode). The IGBT 54 is short-circuited by the initial surge current and a collector current Ic following the initial surge current is generated. Since the collector current Ic following the initial surge current makes charge in the C terminal of the ignition IC 52 (the collector electrode of the IGBT 54) flow to the ground, the potential of the C terminal of the ignition IC 52 is reduced to the potential of the voltage source 59. That is, the protective diode 53 operates as a protective device which protects the IGBT 54 from overvoltage.
In FIG. 5, when the on-signal output from the ECU 51 is input to the G terminal of the ignition IC 52, the gate potential of the IGBT 54 is increased by the resistor 61 and the IGBT 54 is short-circuited. When the IGBT 54 is short-circuited, a primary current flows from the voltage source 59 to the primary coil 57. In contrast, when the off-signal output from the ECU 51 is input to the G terminal of the ignition IC 52, the IGBT 54 is opened and the potential of the C terminal of the ignition IC 52 increases. The voltage of the primary coil 57 also increases. When the flow of the primary current to the primary coil 57 is cut off, a high voltage is generated from the secondary coil 58 according to the turn ratio of the secondary coil 58 and the primary coil 57 and discharge occurs in the gap of the ignition plug 60 (the gap between the electrodes). A spark is generated by the discharge and the fuel-air mixture in a fuel chamber is ignited.
The protective diode 53 is connected between the collector and gate of the IGBT 54. When a high voltage is generated from the collector of the IGBT 54 at the time the flow of the primary current to the primary coil 57 is cut off, the gate potential of the IGBT 54 is increased by a current which flows at the clamping voltage of the protective diode 53 and the resistor 61. Therefore, it is possible to short-circuit the IGBT 54 and absorb a large amount of energy stored in the ignition coil 56. In addition, when a high voltage is applied to the collector of the IGBT 54, the protective diode 53 protects the IGBT 54 such that the IGBT 54 is not broken down.
Next, the IGBT 54 provided with the protective diode 53 will be described. There is a concern that a surge voltage will be applied to a semiconductor device, such as the IGBT 54 which is a power semiconductor element provided in the internal-combustion engine ignition device 700 illustrated in FIG. 5, due to some causes. For example, there are a surge voltage which is applied from the outside, a noise voltage, and a surge voltage that is generated by the operation of the IGBT 54 which is a power semiconductor element. Therefore, the protective diode 53 is provided between the collector and gate of the IGBT 54 and an overvoltage is clamped by the protective diode 53 so as not to be applied to the IGBT 54. In this way, the breakdown voltage of the semiconductor device (ignition IC 52) increases.
Next, the cross-sectional structure of a semiconductor device which will be the ignition IC 52 will be described with reference to FIGS. 6A and 6B. FIG. 6A is a plan view illustrating a main portion of the semiconductor device 600 according to the related art, and FIG. 6B is a cross-sectional view illustrating the main portion of the semiconductor device 600 according to the related art. As illustrated in FIGS. 6A and 6B, a pn junction between a p+ layer 72 with high impurity concentration and an n− layer 70 with low impurity concentration is formed on a LOCOS oxide film 5 in an edge termination region 104 of an IGBT 102 (corresponding to the IGBT 54 illustrated in FIG. 5). The p+ layer 72 and the n− layer 70 are made of polysilicon. A plurality of p+ layers 72 and a plurality of n− layers 70 are alternately and repeatedly arranged and are connected in series to each other.
Both ends of a polysilicon layer forming a protective diode 105 are n+ layers 71 with high impurity concentration. The n+ layer 71 comes into contact with the n− layer 70 which is provided at the outermost end of the polysilicon layer. The p+ layers 72, the n− layers 70 and the n+ layers 71 form the protective diode 105 (corresponding to the protective diode 53 illustrated in FIG. 5). The protective diode 105 is connected between the collector and gate of the IGBT 102. The n+ layers 71 provided at both ends of the polysilicon layer forming the protective diode 105 are connected to the collector and the gate of the IGBT 102.
The n− layer 70 with low impurity concentration which forms the protective diode 105 is formed with a width (a width in a direction in which the layers forming the protective diode 105 are arranged in a line; hereinafter, simply referred to as a width) of 2.5 μm by the implantation of arsenic (As) ions into the polysilicon layer formed on the LOCOS oxide film 5 with a dose of, for example, 6×1013 cm−2. Then, a heat treatment is performed on each layer forming the protective diode 105 at the temperature (1000° C. or lower) of a reflow furnace when a boro-phospho silicate glass (BPSG: silicon glass including boron (B) and phosphorus (P)) film 14 is formed on the protective diode 105. When the n− layer 70 is formed with a small width of 2.5 μm, it is possible to reduce operation resistance. Therefore, the following effect is obtained: the effect (clamping effect) of suppressing an increase in the breakdown voltage in a reverse bias application test which simulatively applies a reverse bias in order to verify the state of a semiconductor device when the reverse bias is applied. The reverse bias application test is a simple type of a voltage clamping test in which the voltage applied to the IGBT 102 is repeatedly clamped by the protective diode 105.
In addition, the following technique has been proposed: a protective diode between the gate and source or between the gate and drain of a metal oxide semiconductor field effect transistor (MOSFET: insulated gate field effect transistor) or a protective diode between the gate and emitter or between the gate and collector of an IGBT has a basic structure in which a pn junction between a p layer and an n layer is repeated (for example, see Japanese Patent Documents JP 9-186315 A, JP 8-88354 A, and JP 9-18001 A).
Furthermore, a protective diode with an n+/n−/p−/n−/n+/n−/p−/n−/n+ structure which is inserted between the gate and source of a power MOSFET has been proposed (for example, see Japanese Patent Document JP 2002-43574 A). In JP 2002-43574 A, the protective diode is connected between the gate and source of the MOSFET and is not connected between the drain and gate of the MOSFET. The protective diode having this structure holds a surge voltage applied between the gate and source of the MOSFET with a p−/n−/n+ junction. In addition, in JP 2002-43574 A, a p− layer with low impurity concentration is formed on one surface of a gate oxide film and an n− layer with low impurity concentration and an n+ layer with high impurity concentration which pass through the p− layer in a depth direction are selectively formed in the p− layer.
However, as described above, the protective diode 105 according to the related art illustrated in FIGS. 6A and 6B has the effect (clamping effect) of suppressing an increase in the breakdown voltage in the reverse bias application test which simulates the application of a surge voltage. However, when the clamping voltage is frequently applied, the pn junction between the p+ layer 72 and the n− layer 70 deteriorates locally. Therefore, a leakage current increases and a breakdown voltage failure occurs. This problem will be described in detail with reference to FIGS. 7 and 8.
FIG. 7 is a cross-sectional view illustrating a main portion of the protective diode 105 of the semiconductor device 600 according to the related art illustrated in FIGS. 6A and 6B. FIG. 8 is a characteristic diagram illustrating the relationship (diffusion concentration distribution) between the diffusion depth and impurity concentration of arsenic (As) in the n− layer 70 forming the protective diode 105 illustrated in FIGS. 6A and 6B. When the dopant of the n− layer 70 is arsenic (As), the impurity concentration of the n− layer 70 in the depth direction (on the side closer to the LOCOS oxide film 5 than to the BPSG film 14) is low since arsenic has a small diffusion coefficient, as illustrated in FIG. 8. When the p+ layer 72 which has a higher impurity concentration than the n− layer 70 is formed in the n− layer 70 having this impurity profile, the width of the n− layer 70 is reduced from the surface (the interface between the BPSG film 14 and the protective diode 105) in the depth direction (a direction from the BPSG film 14 to the LOCOS oxide film 5). Therefore, a pn junction surface 89 between the p+ layer 72 and the n− layer 70 is not flat (substantially perpendicular to the main surface of the substrate). When the clamping voltage is frequently applied to the protective diode 105, the electric field is concentrated on the portion (pn junction surface 89) and the leakage current increases, which results in a breakdown voltage failure.
In JP 2002-43574 A, when the protective diode with a breakdown voltage of tens of volts which is inserted between the gate and source of the MOSFET is inserted between the collector and gate of the IGBT, a high breakdown voltage of about 1000 V or more may be required. As a result, the area of each layer forming the protective diode increases (the width of each layer increases) and the overall area of the protective diode increases.
As disclosed in JP 2002-43574 A, when the protective diode has the n+/n−/p−/n−/n+/n−/p−/n−/n+ structure, the spreading of a depletion layer to the n− layer is stopped by the n+ layer which is adjacent to the n− layer. Therefore, it is possible to reduce the width of the n− layer. In contrast, since a stopper layer for stopping the spreading of the depletion layer to the p− layer is not provided, it may be necessary to increase the width of the p− layer and the area of the protective diode increases. In particular, when the protective diode is connected between the drain and gate of the MOSFET or between the collector and gate of the IGBT, it should have a high breakdown voltage and a large number of p− layers with a large width should be included. As a result, the area of the protective diode increases.